MET.TR.225 Forecasts for landing

(a)TREND forecasts shall be issued in accordance with Appendix 1.

(b)The units and scales used in the TREND forecast shall be the same as those used in the report to which it is appended.

(c)The TREND forecast shall indicate significant changes in respect of one or more of the elements: surface wind, visibility, weather phenomena and clouds. Only those elements for which a significant change is expected shall be included. However, in the case of significant changes in respect of cloud, all cloud groups, including layers or masses not expected to change, shall be indicated. In the case of a significant change in visibility, the phenomenon causing the reduction of visibility shall also be indicated. When no change is expected to occur, this shall be indicated by the term ‘NOSIG’.

(1)Surface wind The TREND forecast shall indicate changes in the surface wind which involve:

(i)a change in the mean wind direction of 60° or more, the mean speed before and/or after the change being 10 kt or more;

(ii)a change in mean wind speed of 10 kt or more

(iii)changes in the wind through values of operational significance.

(2)Visibility

(i)When the visibility is expected to improve and change to or pass through one or more of the following values, or when the visibility is expected to deteriorate and pass through one or more of the following values: 150, 350, 600, 800, 1 500 or 3 000 m, the TREND forecast shall indicate the change.

(ii)When significant numbers of flights are conducted in accordance with the visual flight rules, the forecast shall additionally indicate changes to or passing through 5 000 m.

(iii)In TREND forecasts appended to METAR and SPECI, visibility shall refer to the forecast prevailing visibility.

(3)Weather phenomena

(i)The TREND forecast shall indicate the expected onset, cessation or change in intensity of any of the following weather phenomena or combinations thereof:

(A)freezing precipitation;

(B)moderate or heavy precipitation, including showers thereof;

(C)thunderstorm, with precipitation;

(D)dust storm;

(E)sandstorm;

(F)other weather phenomena as agreed by the aerodrome meteorological office with the ATS units and operators concerned.

(ii)The TREND forecast shall indicate the expected onset or cessation of any of the following weather phenomena or combinations thereof:

(A)freezing fog;

(B)low drifting dust, sand or snow;

(C)blowing dust, sand or snow;

(D)thunderstorm (without precipitation);

(E)squall;

(F)funnel cloud (tornado or waterspout).

(iii)The total number of phenomena reported in points (i) and (ii) shall not exceed three.

(iv)The expected end of occurrence of the weather phenomena shall be indicated by the abbreviation ‘NSW’.

(4)Clouds

(i)When the height of base of a cloud layer of BKN or OVC extent is expected to lift and change to or pass through one or more of the following values, or when the height of base of a cloud layer of BKN or OVC extent is expected to lower and pass through one or more of the following values: 100, 200, 500, 1 000 and 1 500 ft (30, 60, 150, 300 and 450 m), the TREND forecast shall indicate the change.

(ii)When the height of base of a cloud layer is below or is expected to fall below or rise above 1 500 ft (450 m), the TREND forecast shall also indicate changes in cloud amount from FEW, or SCT increasing to BKN or OVC, or changes from BKN or OVC decreasing to FEW or SCT.

(iii)When no clouds of operational significance are forecast and ‘CAVOK’ is not appropriate, the abbreviation ‘NSC’ shall be used.

(5)Vertical visibility When the sky is expected to remain or become obscured and vertical visibility observations are available at the aerodrome, and the vertical visibility is forecast to improve and change to or pass through one or more of the following values, or when the vertical visibility is forecast to deteriorate and pass through one or more of the following values: 100, 200, 500 or 1 000 ft (30, 60, 150 or 300 m), the TREND forecast shall indicate the change.

(6)Additional criteria The aerodrome meteorological office and the users may agree on additional criteria to be used, based on local aerodrome operating minima.

(7)Use of change groups

(i)When a change is expected to occur, the TREND forecast shall begin with one of the change indicators ‘BECMG’ or ‘TEMPO’.

(ii)The change indicator ‘BECMG’ shall be used to describe forecast changes where the meteorological conditions are expected to reach or pass through specified values at a regular or irregular rate. The period during which, or the time at which, the change is forecast to occur shall be indicated using the abbreviations ‘FM’, ‘TL’ or ‘AT’, as appropriate, each followed by a time group in hours and minutes.

(iii)The change indicator ‘TEMPO’ shall be used to describe forecast temporary fluctuations in the meteorological conditions which reach or pass specified values and last for a period of less than 1 hour in each instance and, in the aggregate, cover less than one half of the period during which the fluctuations are forecast to occur. The period during which the temporary fluctuations are forecast to occur shall be indicated using the abbreviations ‘FM’ and/or ‘TL’, as appropriate, each followed by a time group in hours and minutes.

(8)Use of the probability indicator The indicator ‘PROB’ shall not be used in TREND forecasts.